PROJECT SUMMARY Recently, a seemingly novel innate immune cell subset bearing features of natural killer (NK) and B cells was identified in mice. So-called NKB cells appear as first responders to viral infections at mucosal surfaces, but the full functional niche remains unknown and prior to our recent studies it has been unclear if these cells existed in higher order primates. In our recently published findings (Manickam et al., AIDS 2018) and new preliminary data we found that phenotypically analogous populations of NKB cells can be found systemically in humans and non-human primates, but are enriched in spleen, similar to mice. Furthermore, NKB cells had stable surface expression of non-secreted IgM and IgA, and are massively expanded in colonic tissues during chronic SIV infection. In this new proposal we will rigorously test we will test the overarching hypothesis that NKB cells are early responders at mucosal sites of infection and activation of this cell type is critical to initiate anti- HIV/SIV innate immune responses through two focused specific aims: (1) Identify transcriptomic and functional signatures of NKB cells under homeostatic conditions; and (2) Identify the role(s) and functional significance of mucosal NKB cells in SIV infection. The outcomes of this proposal will provide the first detailed analyses of NKB cells during lentivirus infection and potentially means to target these unique cells for immunotherapeutics or vaccine strategies.